In the production of pneumatic tires, it is conventional practice to mold the uncured (commonly referred to as the "green") tire in a molding press in which the tire is pressed outwardly against an outer mold. The green tire is pressed outwardly to mold the tread pattern and shape the sidewalls. The green tire is pressed outwardly by means of a rubber curing bladder which is sequentially pressurized with low pressure steam, followed by high pressure steam or hot water. The bladder is pressed against the inside of the tire for a time sufficient to vulcanize the rubber tire. Once the tire is vulcanized, the pressure in the bladder is reduced and the bladder collapses away from the tire. The tire is then removed from the molding press.
Before a tire is loaded in a curing press, the inside surface of the tire is coated with what is normally referred to as an "inside green tire paint". One of the principle functions of the inside green tire paint is to act as a lubricant between the tire inner liner and the curing bladder both during the loading or shaping stage and the stripping stage of the molding operation. Lubricity is particularly needed during inflation of the bladder in the shaping stage because there is a substantial relative movement between the contacting surfaces of the bladder and the tire inner liner. Unless there is adequate lubrication provided between the tire and inner liner, there is a tendency for the bladder to buckle, which may result in misshaping of the tire and consequent rejection of the tire. At the end of the molding operation when the bladder is collapsed and the tire is stripped from the bladder, there is again considerable relative movement between the contacting surfaces of the bladder and the now cured tire inner liner. Unless adequate lubrication is provided between the bladder and inner liner, the bladder tends to stick to the tire. This causes excessive wear and roughening of the bladder, which results in reduced bladder life. When the bladder sticks to the tire, it also may cause a delay in the molding operation.
Another of the principal functions of the inside green tire paint is to avoid entrapment of major air bubbles between the tire inner liner and the bladder at the beginning of the shaping operation and to promote entry of air between the bladder and the tire inner liner at the end of the molding operation to avoid adhesion of the tire inner liner to the bladder when the bladder is evacuated prior to withdrawal from within the tire. The entrapment of air between the bladder and the inner liner and failure of the liner to separate from the bladder on evacuation of the bladder at the end of the molding cycle may both lead to such severe defects in the molded tire as to require it to be rejected.
The inside green tire paints commonly used heretofore in the rubber industry and more particularly the tire industry, are compositions containing a hydrocarbonsolvent and silicone. U.S. Pat. No. 3,507,247 discloses a typical prior art inside green tire paint composition consisting of ground mica, clay, alcohol, water and silicone, with gasoline as a diluent.
Silicone, although a very effective release agent, can cause defects in the rubber article. If during the vulcanization process, the rubber flows and folds over on itself, then the silicone will prevent the surfaces from adhering. For this reason, it would be desirable if silicone could be eliminated from the inside green tire paints.
Hydrocarbon solvents, such as gasoline, have been used for years as the diluent in these paints. Because of environmental regulations and the cost of hydrocarbon solvents, it would also be desirable to remove them from the inside green tire paints.